The present invention relates generally to gas turbine engines, and more particularly to a bearing mounting system for supporting a low pressure turbine in a gas turbine engine. A gas turbine engine typically includes a high pressure spool, a combustion system and a low pressure spool disposed within an engine case to form a generally axial, serial flow path about the engine centerline. The high pressure spool includes a high pressure turbine, a high pressure shaft extending axially forward from the high pressure turbine, and a high pressure compressor connected to a forward end of the high pressure shaft. The low pressure spool includes a low pressure turbine, which is disposed downstream of the high pressure turbine, a low pressure shaft, which typically extends coaxially through the high pressure shaft, and a low pressure compressor connected to a forward end of the low pressure shaft, forward of the high pressure compressor. The combustion system is disposed between the high pressure compressor and the high pressure turbine and receives compressed air from the compressors and fuel provided by a fuel injection system. A combustion process is carried out within the combustion system to produce high energy gases to produce thrust and turn the high and low pressure turbines, which drive the compressors to sustain the combustion process.
Compressors and turbines are comprised of alternating stages of blades and vanes that are arranged radially around a center axis of the engine within the axial flow path of the engine case. For example, in the low pressure turbine, blades are connected to the low pressure shaft such that they rotate about the engine centerline, while vanes are supported by the engine case such that they remain stationary between the blades. Engine efficiency depends greatly on the ability of the engine to pass airflow through the blades and vanes within the axial flow path without leakage. To increase efficiency, the radially outer end of the blades are designed to come into close proximity with the stationary engine case as the high energy gases from the combustion process rotate the turbines. Typically, a sealing system is provided to reduce leakage from around the tips of the blades. For example, an abradable seal is positioned on the inner diameter surface of the engine case above the low pressure turbine, and a knife edge is positioned on the tip of each of the low pressure turbine blades to seal the air flow path. The ability of this sealing system to function depends on the consistency with which the low pressure shaft rotates about the engine centerline.
In many conventional engines, the low pressure turbine is typically supported within the engine case by a support rotor connected near the aft end of the low pressure turbine. Typically, the aft end of the low pressure shaft is supported by a single bearing, often referred to as the number five bearing, positioned either forward or aft of the support rotor. The use of a single number five bearing, however, can result in rotor dynamics issues such as turbine instability and vibration. For example, a single bearing support can lead to forward or rearward tilting of the support rotor. Rotor tilt causes the turbine blades and the knife edge seals to pull away from the abradable material and the engine case, causing an air leak and decreasing engine efficiency. One factor contributing to rotor tilt is large amounts unsupported shaft length, which results from using a single bearing to support the low pressure turbine. Large unsupported shaft lengths not only increase shaft vibration, but also reduce the maximum rotational speed, or critical speed, of the shaft, thus limiting the operational speeds of the engine. Critical speed can be increased by increasing the diameter of the shaft, which unfavorably increases shaft weight and area.
Previous attempts have been made to support low pressure turbines using more than one bearing, but have resulted in overly complex systems that do not address rotor dynamics issues for turbines having a single support rotor. For example, U.S. Pat. No. 5,074,109 to Mandet et al. discloses a low pressure turbine having a rotor drum supported by two transverse elements, and a shaft supported by two bearings. Also, U.S. Pat. No. 5,813,214 to Moniz et al. discloses a low pressure turbine supported by a single rotor, an aft bearing supporting the low pressure shaft and a forward bearing supporting the high pressure shaft. There is, therefore, a need for a mounting system that improves rotor dynamics and engine efficiency in low pressure spool systems without increasing size, weight and complexity of the system.